1. Field of the Invention
The present invention relates to an RF (radio frequency) modulator for transmission of an audio signal and a video signal through frequency-division multiplexing, and more particularly to a modulation circuit and a PLL (phase-locked loop) circuit relative thereto in a compact miniature RF modulator adapted for use in VHF-UHF television.
2. Description of the Prior Art
There is known a conventional example of an RF converter as disclosed in U.S. Pat. No. 4,654,607. First the operation of such exemplary converter will be described below with reference to FIG. 1 which shows the circuit configuration thereof.
The above modulator comprises bipolar transistors. A video signal e.sub.V ' is supplied to both bases of transistors Q5 and Q6 of a differential amplifier 6, and the output thereof is supplied as a signal current to a common emitter junction of paired differential transistors Q1 and Q2 which constitute a double balanced modulator.
Meanwhile a frequency-modulated (FM) audio signal e.sub.A is supplied to both bases of transistors Q7 and Q8 constituting a differential amplifier 7, and the output thereof is supplied as a signal current from a collector of the transistor Q7 to a common emitter junction of paired differential transistors Q4 and Q5 constituting a double balanced modulator.
Signals of modulation frequencies fc, fc are supplied respectively to the bases of paired differential amplifier transistors 4, 5 of such double balanced modulator and are amplitude-modulated by the video signal e.sub.V ' and the FM audio signal.
The outputs of the paired differential transistors 4 and 5 are combined with each other and, after being amplified by a buffer amplifier 3, the combined output is taken out as an AM carrier signal from an output terminal 2.
For frequency modulation of an audio signal, it is generally customary to employ a voltage controlled oscillator (VCO).
In the VHF band, an oscillator with an SAW resonator is used for generating the carrier signals fc, fc which are to be modulated by the video signal e.sub.V ' and the FM audio signal e.sub.A. Meanwhile in the UHF band, it is usual to use an LC oscillator with a coaxial resonant circuit consisting of a variable capacitor.
The modulator for use in a VHF video tape recorder (VTR) is so designed as to be switchable merely to two channels due to employment of an SAW resonator.
Meanwhile the UHF RF modulator is designed to be adjustable to a selected channel with control of a variable capacitor due to employment of a coaxial resonant circuit. Each of such two types is operated manually.
Although the RF converter mentioned above is adoptable for transmission of an audio signal and a video signal through frequency-division multiplexing, no contrivance is proposed with regard to realization of an IC (integrated circuit) configuration.